1. Field of the Invention
The present invention relates generally to apparatus and methods for measuring the frequency of an input signal, and more particularly for measuring the frequency of an input signal for a tachometer.
2. Discussion of Background Art
Automotive tachometers are mass market devices designed for overall functionality and low cost. Since cost is typically the driving factor in tachometer design and only a reasonable degree of accuracy is typically required, many automotive tachometers are comprised of a minimum number of components. An added benefit of having very few components is a tachometer that also requires fewer silicon resources.
Low cost frequency measuring circuits, however, have performance limitations. Many circuits, for instance, non-linearly translate frequency pulses into a frequency value. As a result, a display must be modified to compensate for the non-linearly increasing frequency values. In an analog display, this is often compensated for by adjusting spacing between gauge marks painted on the analog display. The resultant display then has gauge marks whose spacing is non-uniform.
Many low-cost frequency measuring circuits also suffer from a "ratcheting effect" under deceleration conditions. This ratcheting effect is noticeable as a series of needle jumps on an analog display, instead of a smooth continuous movement as would be expected by one viewing the display.
What is needed is a tachometer circuit that overcomes the limitations of the prior art described above.